1. Field of the Invention
The present invention relates to a cathode-ray tube having an improved shadow mask adapted to effectively suppress spring-back phenomena occurring in the skirt portion thereof.
2. Background Information
A shadow mask is typically suspended within a cathode-ray tube envelope and positioned adjacently to a phosphor screen on the inner surface of a panel. The shadow mask functions as a color selection electrode which filters the scanning electron beams projected from an electron gun, incorporated in the neck of a funnel, toward the phosphor screen.
The conventional shadow mask 10 depicted in FIG. 1 comprises central apertured area 12 having a pattern of electron beam transmissive apertures 11, unapertured border 13 surrounding central apertured area 12, skirt portion 13a formed along the periphery of unapertured border 13 and positioned substantially perpendicular to central apertured area 12, and ridge 14 formed along the periphery of the central apertured area 12 in unapertured border 13. Ridge 14 is provided for reinforcing shadow mask 10. Skirt portion 13 serves as a welding portion for attaching shadow mask 10 to frame 20.
Shadow mask 10 has a scooped surface as shown in FIG. 2 and is formed by a press-forming process using a flat thin blank of metal. The press-forming process is accomplished by drawing the blank of metal consisting of central apertured area 12 together with unapertured border 13 into a domed shape and bending the periphery of unapertured border 13 to form skirt portion 13a. Here, the aforesaid steps are successively performed by one press machine.
However, a shadow mask formed by this press-forming process has a problem. A spring-back phenomena generally occurs in the shadow mask because a complete plastic deformation is not achieved in press-forming the bent portion.
The spring-back phenomena occurs chiefly in the skirt portion making it difficult to accurately weld the shadow mask to a frame because there is loose contact between the two surfaces.
When the skirt portion and the frame are widely gaped and forcibly welded together, the skirt portion becomes partially deformed causing the central apertured area and the unapertured border to become distorted.
The central apertured area which has a plurality of electron beam passing apertures is extremely fragile in structure in comparison with the unapertured border or the skirt portion. The central apertured area is therefore easily distorted causing the electron beam transmissive apertures to become misarranged. As a result, the filtering of electron beams through the electron beam transmissive apertures is not accurately performed, thereby degenerating the color purities of the screen image formed on the phosphor screen.